One tweet reads: ‘If the United States bomb Iraq, every American citizen is a legitimate target for us.’

Some pro-ISIS social media users have even made film poster-style mock-ups of cities like New York in flames.

The tweets began the day after the U.S. began bombing ISIS militant targets outside the Kurdish city of Irbil in the first offensive action by the United States in the region since it withdrew ground troops in 2011.

The U.S. said militants were shelling Kurdish forces defending the city and that American personnel there are at risk.

The new messages highlight the possible consequences of President Barack Obama’s decision to authorise military action against ISIS, many of whom are armed with high-tech weapons initially given to the Iraqi army by the U.S. in the decade since the inception of the Iraq war.

‘All it would take is one attack on a diplomatic facility to rally more online strutting,’ an anonymous State Department official told the MailOnline.

‘Lots of people will blame the president for antagonizing people who already want to kill Americans.’